The present invention relates to an insulation detecting device in use for a non-grounded power source, more particularly, to an insulation detecting device which is well adaptable for a non-grounded DC power source carried on a vehicle of the type which is driven by a propelling force electrically generated.
A related insulation detecting device in use for the non-grounded power source is disclosed in JP-A-08-226950 (pages 4–7, FIG. 1), for example. The insulation detecting device detects to a ground resistance, i.e., an insulation resistance with respect to a ground potential part of the positive and negative main circuit wirings which are connected to the positive and negative terminals of a non-grounded DC power source and insulated from a ground potential part. The insulation detecting device includes a first switching unit for connecting a capacitor to between a positive terminal of the non-grounded DC power source and a ground potential part in series for a preset time, a second switching unit for connecting the capacitor to between a negative terminal of the non-grounded DC power source and a ground potential part in series for a preset time, a third switching unit for connecting a voltage detecting unit for detecting voltage across the capacitor after the respective switching units are turned off, and a computing unit for computing an insulation resistance, i.e., a ground resistance, between the power source and the ground potential part by using the voltage across the capacitance which is detected by the voltage detecting unit after the switching units are turned off and a power source voltage previously computed in a state that the capacitor is completely charged. The insulation detecting device detects or decides an insulation state of the main circuit wirings by using the ground resistance computed by the computing unit.
To compute the ground resistance, the insulation detecting device uses a formula containing a capacitance value of the capacitor as a constant. The capacitance of the capacitor varies in value among capacitor products and when temperature varies. Further, the capacitance value sometimes varies due to aging of the capacitor. Where the capacitance value used as the constant varies or changes, an error of a computed ground resistance value to an actual resistance value increases. This deteriorates an accuracy of detecting an insulation state of the main circuit wirings to a ground potential part. For this reason, there is a demand of developing new techniques which are capable of minimizing the measurement error of the ground resistance and enhancing the accuracy of the insulation state detection even if there is a variation in the value of a circuit part, such as a capacitance value, which is used as a constant in the formula in computing the ground resistance.